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Dynamics of the seroprevalence of SARS-CoV-2 antibodies among healthcare workers at a COVID-19 referral hospital in Milan, Italy
  1. Laura Milazzo1,
  2. Alessia Lai2,
  3. Laura Pezzati2,
  4. Letizia Oreni1,
  5. Annalisa Bergna2,
  6. Federico Conti2,
  7. Cristina Meroni1,
  8. Davide Minisci1,
  9. Massimo Galli2,
  10. Mario Corbellino1,
  11. Spinello Antinori2,
  12. Anna Lisa Ridolfo1
  1. 1 Department of Infectious Diseases, ASST Fatebenefratelli Sacco, Milano, Italy
  2. 2 Luigi Sacco Department of Biomedical and Clinical Sciences, Università degli Studi di Milano, Milano, Lombardia, Italy
  1. Correspondence to Dr Laura Milazzo, Infectious Diseases, ASST Fatebenefratelli Sacco, Milano, Italy; laura.milazzo{at}


Objectives Healthcare workers (HCWs) are at high risk of developing SARS-CoV-2 infection. The aim of this single-centre prospective study was to evaluate the trend of SARS-CoV-2 seroprevalence in HCWs working at the primary referral centre for infectious diseases and bioemergencies (eg, COVID-19) in Northern Italy and investigate the factors associated with seroconversion.

Methods Six hundred and seventy-nine HCW volunteers were tested for anti-SARS-CoV-2 antibodies three times between 4 March and 27 May 2020 and completed a questionnaire covering COVID-19 exposure, symptoms and personal protective equipment (PPE) training and confidence at each time.

Results SARS-CoV-2 seroprevalence rose from 3/679 to 26/608 (adjusted prevalence: 0.5%, 95% CI 0.1 to 1.7% and 5.4%, 95% CI 3.6 to 7.9, respectively) between the first two time points and then stabilised, in line with the curve of the COVID-19 epidemic in Milan. From the first time point, 61.6% of the HCWs had received training in the use of PPE and 17 (61.5%) of those who proved to be seropositive reported symptoms compatible with SARS-CoV-2 infection. Contacts with ill relatives or friends and self-reported symptoms were independently associated with an increased likelihood of seroconversion (p<0.0001 for both), whereas there was no significant association with professional exposure.

Conclusion The seroprevalence of SARS-CoV-2 among the HCWs at our COVID-19 referral hospital was low at the time of the peak of the epidemic. The seroconversions were mainly attributable to extrahospital contacts, probably because the hospital readily adopted effective infection control measures. The relatively high number of asymptomatic seropositive HCWs highlights the need to promptly identify and isolate potentially infectious HCWs.

  • occupational health
  • viruses
  • health personnel
  • longitudinal studies
  • personal protective equipment

Data availability statement

Data are available on reasonable request. Anonymised participants data were collected in a protected database and are available on reasonable request from LM ( and ALR (

This article is made freely available for use in accordance with BMJ’s website terms and conditions for the duration of the covid-19 pandemic or until otherwise determined by BMJ. You may use, download and print the article for any lawful, non-commercial purpose (including text and data mining) provided that all copyright notices and trade marks are retained.

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Data availability statement

Data are available on reasonable request. Anonymised participants data were collected in a protected database and are available on reasonable request from LM ( and ALR (

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  • Contributors LM and AL designed the study and wrote the manuscript; AL and AB performed the serological analyses; LP, FC and DM carried out the survey on the healthcare workers (HCWs); CM organised the communication to the HCWs and their periodical calls; LO performed statistical analysis; MC was involved in planning the conceptual idea; MG and SA supervised the work and critically revised the manuscript.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.